1. Field of the Invention
This invention relates to a novel and improved anti-lock control system for motor vehicles, which is operative to prevent the wheels of the motor vehicle from being locked during a braking operation of the motor vehicle.
2. Description of the Prior Art
Generally, with an anti-lock control system for motor vehicles, anti-lock control is effected by means of microcomputers such that hold valves and decay valves comprising electromagnetic valves are opened and closed on the basis of electrical signals representing wheel speeds sensed by wheel speed sensors, thereby increasing, holding or reducing the brake hydraulic pressure, for the purpose of securing improved steering performance and running stability of the motor vehicle, while at the same time shortening the braking distance.
FIG. 1 of the accompanying drawings illustrates, by way of example, ways in which the wheel speed Vw, wheel acceleration and deceleration dVw/dt and brake hydraulic pressure Pw are varied during the operation of the conventional anti-lock control system, together with hold signal HS and decay signal DS for opening and closing the hold valves and decay valves, as disclosed in U.S. Pat. No. 4,741,580.
When the brake equipment of the motor vehicle is not being operated while the motor vehicle is running, the hold valves remain open while the decay valves remain closed, the brake hydraulic pressure Pw is not increased; and when the brake equipment is operated, the brake hydraulic pressure Pw is rapidly increased at time t0 so that the wheel speed Vw is decreased (normal mode). A reference wheel speed Vr is set up which is lower by a predetermined amount .DELTA.V than the wheel speed Vw and follows the latter with such a speed difference. More specifically, the reference wheel speed Vr is set up so that when the deceleration (negative acceleration) dVw/dt of the wheel reaches a predetermined threshold level, say -1.1G at time t1, an anti-lock control is started, and the reference wheel speed Vr is thereafter made to decrease linearly with a deceleration gradient .theta. (=-1.1G). At time t2 when the deceleration dVw/dt of the wheel reaches a predetermined maximum value -Gmax, a hold signal HS is generated so that the hold valves are closed, thus holding the brake hydraulic pressure Pw.
While the brake hydraulic pressure Pw is being held constant, the wheel speed Vw is further decreased. At time t3, the wheel speed Vw and the reference wheel speed Vr become equal to each other, and a first cycle of anti-lock control is started; and a decay signal DS is generated, by which the decay valves are opened so that a reduction of the brake hydraulic pressure Pw is started. As a result of this reduction of the brake hydraulic pressure Pw, the wheel speed Vw is changed from increase to decrease at time t4 when a low peak VL of the wheel speed Vw occurs. At time t4, the decay signal DS is interrupted as shown in FIG. 1, and as a result the decay valves are closed so that the reduction of the brake hydraulic pressure Pw is stopped and thus the brake hydraulic pressure is held constant. The wheel speed Vw is further increased up to the level of a speed Vc which is higher than the low peak speed VL by 80% of a difference Y between the wheel speed Va occurring at the time t3 when the reduction of the brake hydraulic pressure Pw was started and the low peak speed VL, i.e., Vc=VL+0.8Y. Subsequently, at time t7, a high peak VH of the wheel speed Vw is reached; thereupon, the brake hydraulic pressure Pw is again increased. In this case, the buildup of the brake hydraulic pressure Pw is effected in such a manner that the brake hydraulic pressure Pw is alternately increased and held in succession by the fact that the hold signal HS is turned on and off minutely, i.e., with relatively short intervals so that the brake hydraulic pressure Pw is caused to build up gradually. In this way, the wheel speed Vw is decreased, and at time t8 (corresponding to the time t3), a second cycle of the mode for reduction of the brake hydraulic pressure occurs. The time period Tx of the initial brake hydraulic pressure buildup occurring at the time t7 is determined on the basis of calculation of the average acceleration (Vc-Vb)/.DELTA.T over the time interval .DELTA.T between the time t5 and wherein the time t6 (the average acceleration depends on the friction coefficient .mu. of the road surface), and the time period of the subsequent pressure holding or pressure buildup is determined on the basis of the acceleration or deceleration of the wheel which is detected immediately prior to the pressure holding or pressure buildup. The brake hydraulic pressure increasing, holding and reducing modes are effected in combination as mentioned above, and thus the wheel speed Vw can be controlled so that the vehicle speed can be decreased, while the wheels of the motor vehicle are prevented from being locked.
When the above-mentioned anti-lock control is effected, the wheel speed Vw which is used as a reference speed is computed on the basis of a frequency signal derived from a speed sensor provided in association with the wheel and is subjected to filtering in the range from +30G to -30G for example to prevent faulty operation due to noise. To compute the wheel speed Vw, a predetermined period of time is required so that the computed speed Vw shown by a solid line in FIG. 2 tends to be delayed by a certain time period say 10 to 16 ms with respect to the real wheel speed Vw' shown by a one-dotted chain line. Thus, ideally, the pressure reduction should be interrupted at a time point P' when a low peak of the real wheel speed Vw' occurs, and thereafter the brake hydraulic pressure should be held constant as shown by a one-dotted chain line Pw' in FIG. 2; actually, however, it is at a time point P which is later by a certain time period than the time point P' that the pressure reduction is interrupted so that the brake hydraulic pressure Pw is correspondingly further reduced. In general, the decreasing gradient of the brake hydraulic pressure as the anti-lock control is being effected is higher than the increasing gradient; thus, the brake hydraulic pressure is excessively reduced because of the delayed interruption of the pressure reduction. For a medium- or high-.mu. road surface representing a high friction coefficient .mu., difficulties are encountered in that due to the excessive reduction of the brake hydraulic pressure, the braking distance is increased, and also the variation range of the brake hydraulic pressure is so widened that a vibration is caused to occur in the motor vehicle.